Quantum discord and its geometric measure with death entanglement in correlated dephasing two qubits system

1.3 MB·English

by  Abdel-Baset A. Mohamed

Quantum discord and its geometric measure with
death entanglement in correlated dephasing two
qubits system
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0
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Abdel-Baset. A. Mohamed

n
Faculty of Science, Assiut University, Assiut, Egypt
a
J

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Abstract. The quantum correlations, including entanglement and discord with its
3
geometric measure, and classical correlation are studied for a bipartite partition of

] a open or closed quantum system. It is found that the purity of the initial state
h
plays an important role in the dynamics of quantum and classical correlations. In the
p
- dephasing model, the quantum correlations loss and the classical correlation gain are
t
n instantaneously happen. While, the purity of the initial state destroys the quantum
a
correlations which is resulted by the unitary interaction. Therefore, with the purity
u
q parameter, a particular region in which there is no state have quantum correlations
[ can be determined.

Keywords:Quantum discord; geometric measure of quantum discord; dephasing
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v model
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. 1. Introduction
1
0
3 The development of the quantum information technology stimulates a deep study of
1
: the properties of quantum correlations inherent in a quantum system. In particular,
v
i there is a problem of identification of those quantum correlations which are responsible
X
for the advantages of the quantum computations in comparison with the classical ones.
r
a
Therefore, lots of interest have been devoted to the definition and understanding of
correlations in quantum systems in the last two decades. Specially, the definition and
study of quantum and classical correlations in quantum systems. It is well known
that the total correlation in a bipartite quantum system can be measured by quantum
mutual information [1], which may be divided into classical and quantum parts [2-5].
The quantum part is called quantum discord(QD) which is originally introduced by [4].
Recently, it has been aware of the fact that quantum discord is a more general concept
to measure quantum correlation than quantum entanglement(QE) since there is a
nonzero quantum discord in some separable mixed states [4]. The dynamics of quantum
discord and entanglement has been recently compared under the same conditions when
entanglement dynamic undergoes a sudden death [6, 7, 8, 9, 10]. It was shown that
quantum discord presents an instantaneous disappearance at some time points in non-
Markovian regime [6], and exponential decay in Markovian regime[7]. Interestingly,
Quantum discord and its geometric measure with death entanglement ... 2
it has been proven both theoretically and experimentally that such states provide
computational speedup compared to classical states in some quantum computation
models[11, 12]. In these contexts, quantum discord could be a new resource for quantum
computation.
The calculation of quantum discord is based on numerical maximization procedure,
it does not guarantee exact results and there are few analytical expressions including
special cases [13, 14]. To avoid this difficulty, geometric measure of quantum discord
(GMQD) is introduced by Ref.[15], which measures the quantum correlations through
the minimum Hilbert-Schmidt distance between the given state and zero discord state.
Because of the unavoidable interaction between a quantum system and its
environment, understanding the dynamics of quantum and classical correlations (CC) is
aninteresting lineofresearch[14-17]. Theinfluence ofMarkovian[7]andnon-Markovian
[17] environment on the dynamics of QD and GMQD. They showed that both GMQD
and QD die asymptotically with entanglement sudden death, and the discontinuity in
the decay rate of GMQD does not always imply the discontinuity in the decay rate
of QD. Also, they observed that even when QD vanishes at discrete times, GMQD
disappears but not instantly. But in our work the dephasing environment, in which
energy transfer from the system to the environment does not occur, is considered.
Some work has been devoted to this issue [20, 21, 22], in which, the authors show
that disentanglement is dependent on the initial condition and temperature of the
environment. This type of the environment leads to phenomenon entanglement sudden
death, i.e., the entanglement can decrease to zero abruptly and remains zero for a finite
time [23, 24, 25]. Entanglement sudden death has been experimentally observed in an
implementation using twin photons [26], and atomic ensembles [27].
In this paper, one considers a dephasing model, in which two qubits embed into a
multi-modequantized fieldandtheinteractionbetweenthetwo qubitsisalsoconsidered.
Therefore, the quantum correlations via quantum discord and its geometric measure
(GMQD) is compared with both quantum entanglement and classical correlation.
2. Measures of correlations
2.1. Quantum discord
To quantify the quantum correlations of a bipartite system, no matter whether it is
separable or entangled, one can use the quantum discord [2, 4]. Quantum discord
measures all nonclassical correlations and defined as the difference between total
correlation and the classical correlation with the following expression
D(ρAB) = I(ρAB)−Q(ρAB), (1)
which quantifies the quantum correlations in ρAB and can be further dis-
tributed into entanglement and quantum dissonance (quantum correlations excluding
entanglement)[28]. Here the total correlation between two subsystems A and B of a
Quantum discord and its geometric measure with death entanglement ... 3
bipartite quantum system ρAB is measured by quantum mutual information,
I(ρAB) = S(ρA)+S(ρB)−S(ρAB), (2)
where S(ρAB) = Tr(ρABlogρAB) is the Von Neumann entropy, ρA = Tr (ρAB)
B
and ρB = Tr (ρAB) are the reduced density operators of the subsystems A and B,
A
respectively. The measure of classical correlation is introduced implicitly in Ref.[4]
and interpreted explicitly in the Ref.[2]. The classical correlation between the two
subsystems A and B is given by
Q(ρAB) = max[S(ρA)− p S(ρ )], (3)
X k k
{Πk} k
where {Π } is a complete set of projectors to measure the subsystem B, and ρ =
k k
Tr [(IA⊗Π )ρ (IA⊗Π )]/p is the state of the subsystem A after the measurement
B k AB k k
resulting in outcome k with the probability p = Tr [(IA ⊗ Π )ρAB(IA ⊗ Π )], and
k AB k k
IA denotes the identity operator for the subsystem A. Here, maximizing the quantity
represents the most gained information about the system A as a result of the perfect
measurement {Π }. It can be shown that quantum discord is zero for states with
k
only classical correlations and nonzero for states with quantum correlations. Note
that discord is not a symmetric quantity, i.e., its amount depends on the measurement
performed on the subsytem A or B [15].
2.2. Geometric measure of quantum discord
The geometric measure of quantum discord quantifies the quantum correlation through
the nearest Hilbert-Schmidt distance between the given state and the zero discord state
[15, 16], which is given by
Dg = minkρAB −χk2, (4)
A
χ∈S
where S denotes the set of zero discord states and kAk2 = Tr(A†A) is the square of
g
Hilbert-Schmidt norm of Hermitian operators. The subscript A of D implies that the
A
measurement is taken on the system A. A state χ on HA⊗HB is of zero discord if and
only if it is a classical-quantum state [29], which can be represented as
2
χ = p |kihk|⊗ρ ,
X k k
k=1
where {p } is a probability distribution, |ki is an arbitrary orthonormal basis for HA
k
and ρ is a set of arbitrary states (density operators) on HB. An easily computable
k
exact expression for the geometric measure of quantum discord is obtained by Ref.[15]
for a two qubit system, which can be described as follows. Consider a two-qubit state
ρAB expressed in its Bloch representation as
1
ρAB = [IA ⊗IB + x (σ ⊗IB +IA ⊗y σ )
4 X i i i i
i=1
+ R σ ⊗σ ], (5)
X ij i j
ij=1
Quantum discord and its geometric measure with death entanglement ... 4
where {σ } are the usual Pauli spin matrices. The components of the local Bloch vector
i
are x = Tr(ρAB(σ ⊗ I)) and y = Tr(ρAB(I ⊗ σ )). R = Tr(ρAB(σ ⊗ σ )) are the
i i i i ij i j
components of the correlation matrix[15]. Therefore, its geometric measure of quantum
discord is given by
1
Dg = (k~xk2 +kyk2 −k ), (6)
A 4 max
~x = (x ,x ,x )T, R is the matrix with elements R and k is the largest eigenvalue
1 2 3 ij max
of the matrix K = ~x~xT +RRT.
2.3. Entanglement via Negativity
Here, one uses the negativity[30] to measure the entanglement, i.e., the negative
eigenvalues of the partial transposition of ρAB are used to measure the entanglement of
the qubits system. Therefore, the negativity of a state ρAB is defined as
N(ρ) = max(0,−2 µ ), (7)
X j
j
where µ is the negative eigenvalue of (ρAB(t))TB, and T denotes the partial transpose
j B
with respect to the second system.
3. Quantum and classical correlations in two-qubit models
In this section one tries to present, by examining two examples, an physical
interpretation for the relation between quantum entanglement, quantum and classical
correlations. The first model consists of two non-interaction qubits couple with the
same quantized field under the rotating-wave approximation. Another is the dephasing
model, in which two qubits embed into a multi-mode quantized field and the interaction
between the two qubits is also considered.
3.1. Two non-interaction qubits couple with the field
Here, one considers two qubits coupled with a single-mode cavity field, which is in
the case of the exact resonance with the qubits. One of the pioneering potential
applications on this Hamiltonian in the context of quantum information is ” Cooper
pair box”(qubits)[31], i.e., the coupled system of two Cooper pair box (artificial atoms)
and photons stored in the resonator (cavity mode). The cavity is sustaining non-
decaying single mode field in its thermal state along with the mode structure of the
electromagnetic field. The interaction picture Hamiltonian with the rotating wave
approximation is given by
2
Hˆ = λ (aˆ†|0i h1|+aˆ|1i h0|), (8)
X k k
k=1
where aˆ† and aˆ are the creation and annihilation operators for the cavity mode, |0i
k
and |1i denote to the ground and excited states of the k-th qubit, respectively. For the
k
Quantum discord and its geometric measure with death entanglement ... 5
whole system (system+field), the evolution of the whole system is characterized by the
interaction between the two-qubits system and single-mode cavity. However from the
point of the qubits, the energy transfer between the qubits and the field happens, which
is described by the relaxation term aˆ†|0i h1| and the backaction term aˆ|1i h0|. One
k k
assumes that the two qubits are initially prepared in Werner states, which is defined by
1
ρAB(0) = p|ϕihϕ|+ (1−p)Iˆ, (9)
4
where |ϕi = sinθ|11i + cosθ|00i and p is a real number which indicates the purity
of initial state, Iˆ is a 4 × 4 identity matrix. But the cavity field is initially prepared
in the vacuum state, i.e., ρF(0) = |0ih0|. Then the initial density operation for the
whole qubits-field system is: ρABF(0) = ρAB(0) ⊗ |0ih0|. By using the above initial
states, the density matrix of the qubits-field system with the interaction (8) evolves to
ρ(t) = Uˆ(t)ρABF(0)Uˆ†(t), where the time evolution operator Uˆ(t) = exp(−iHˆt). The
reduced density matrix, ρAB(t), of two qubits is calculated by tracing out the cavity
field variables. Therefore, ρAB(t) is given by
ρAB(t) = a |11ih11|+a |00ih00|+a (|11ih00|
1 2 3
+|00ih11|)+a (|10ih10|+|01ih01|)
4
+a (|10ih01|+|01ih10|) (10)
5
with the abbreviation
1−p p
a = + (2+cos̟t)2sin2θ,
1
4 9
1−p 2p
a = +pcos2θ + (1−cos̟t)2sin2θ,
2
4 9
p
a = (2+cos̟t)sinθcosθ,
3
3
1−p p 1−p
a = + sin2̟tsin2θ, a = a − ,
4 5 4
4 6 4
where ̟ = 2.4495λ. The eigenvalues of the density matrix ρAB(t) are given by:λ =
1,2
a ± a , and λ = 1[(a + a ) ± (a −a )2 −4a2]. After some straightforward
4 5 3,4 2 1 2 q 1 2 3
calculation, the reduced density matrices associated with the above states is given by
ρA(t) = ρB(t)
= (a +a )|11ih11|+(a +a )|00ih00|. (11)
1 4 2 4
One noted that, the reduced density matrices of the qubits are represented in diagonal
matrices. Therefore, these the states ρA(t) and ρB(t) are classical states.
For the case of the two qubits coupled with a single-mode cavity field, the results
are given in Figs.1a,b. In these figures, one reports the dynamics of Dg, D(ρAB), N(ρ)
A
and Q(ρ) as a functions of the time λt for different values of the purity of initial state
(namely p = 0.5,1.0) with θ = π. It is worth noting to mention that, because the
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matrix of the initial state of the two qubits is not a diagonal matrix, this state is not
classical state. Therefore, its quantum correlation have non zero value at λt = 0.0.
For p = 1.0, these measures instantaneously oscillate and reach their maximum (at
Quantum discord and its geometric measure with death entanglement ... 6
1.1 1.1
(a) (b)
1 1
0.9 0.9
0.8 0.8
0.7 0.7
0.6 0.6
0.5 0.5
0.4 0.4
0.3 0.3
0.2 0.2
0.1 0.1
0 0
0 1.2825 2.565 3.8475 5.135.5 0 1.2825 2.565 3.8475 5.135.5
λ t λ t
Figure1. Timeevolutionsofthequantumdiscord(dashplots),thegeometricmeasure
ofQD(soldplots),thenegativity(dottedplots)andtheclassicalcorrelation(dash-dot
plots) for p=1 in (a) and p=0.5 in (b) for θ = π.
4
λt = 2nπ,n = 0,1,2,...) and minimum values (at λt = (2n+1)π) at the same time points
ω ω
(see Fig.1a). Because the function cos̟t is a periodical function on the scaled time
with period 2π, Dg, D(ρ), N(ρ) and Q(ρ) evolve periodically with respect to the scaled
̟ A
time with period 2π (see Figs.1a,b). A rather counterintuitive feature of the QE is that
̟
it may exceed the measures of the GMQD and QD (see Fig.1a). So one can say that
GMQD and QD are more general than QE.
For p = 0.5 (see Fig.1b), one can observe that the phenomenon of entanglement
death occurs, but this phenomenon does not occur for GMQD and QD even when
the purity p is small. Because the entanglement undergoes sudden death while the
correlations are long lived, QE is not greater than QD, GMQD and CC for some
time. Also one can see that one value of QE corresponds to many values of QD
and GMQD, meaning that the states in possession of the same entanglement give
different correlations. This means that, there are correlations(quantum and classical)
in the intervals of entanglement death. Therefore, the entanglement is not the only
part of quantum correlations. This agrees with Ref.[29], which showed that absence of
entanglement does not imply classicality. From Fig.1a, one can see that the intervals of
vanishing negativity disappear when p = 1. Therefore, the entanglement sudden death
is completely sensitive to the purity of the initial-state.
3.2. Dephasing two interaction qubits by a multimode quantized field
Here, one considers a dephasing model of two qubits embed into a multimode quantized
field and the interaction between the two qubits is also considered. The dephasing
channel case is important situation for the open systems, in which there is no energy
transfer between the system and environment. The Hamiltonian of this case can be
Quantum discord and its geometric measure with death entanglement ... 7
0.3(a)
0.25
0.2
0.15
0.1
0.05
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
λ t
0.3(b)
0.25
0.2
0.15
0.1
0.05
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
λ t
Figure2. Timeevolutionsofthequantumdiscord(dashplots),thegeometricmeasure
ofQD(soldplots),thenegativity(dottedplots)andtheclassicalcorrelation(dash-dot
plots) for γ =0.0λ in (a) and γ =1.0λ in (b)for θ = π and p=0.5.
60
written as[33]
ω
Hˆ = 0(σz +σz)+λ(σ+σ− +σ+σ−)
2 A B A B B A
+ ω ˆb†ˆb +γ (σ+σ− +σ+σ−)(ˆb† +ˆb ), (12)
X k k k k A B B A k k
k
where ω is the qubit transition frequency and ω the frequency of the k-th field,
o k
the coupling constant between the two qubits is λ but γ is system-reservoir coupling
k
constant (dephasing channel parameter).
By using the same previous initial states but with |ϕi = sinθ|10i + cosθ|01i,
the density matrix of the dephasing qubits-field system is given by: Uˆ(t)ρABF(0)Uˆ†(t).
Therefore, the reduced density matrix of the two qubits is given by
ρAB(t) = α (|11ih11|+|00ih00|)+α |10ih10|
1 2
+α |01ih01|+α |10ih01|+α∗|01ih10|, (13)
3 4 4
with the abbreviation
α = (1−p)/4, α = (1+p)/4−β,
1 2
Quantum discord and its geometric measure with death entanglement ... 8
0.3(a)
0.25
0.2
0.15
0.1
0.05
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
λ t
0.3(b)
0.25
0.2
0.15
0.1
0.05
0
0 0.2 0.4 0.6 0.8 1
λ t
Figure 3. The same as in Fig.2 but for (γ,θ)=(2, π ) in (a) and (γ,θ)=(2,π) in
λ 60 λ 3
(b)
p
α = (1+p)/4+β, α = sin2θ−iβ,
3 4
2
p
β = L cos2θcos2λt,
d
2
γ
L = exp[−4 ( k)2(1−cosω )].
d X ω k
k k
Where L is the decoherence factor which leads to damping of the off-diagonal terms.
d
One can get the eigenvalues of the density matrix ρ(t) as
1
λ = α , λ = [1+p± (1+p)2 −16|α |2] (14)
1,2 1 3,4 q 4
4
The reduced density matrices associated with the above states are given by
ρA(t) = (α +α )|11ih11|+(α +α )|00ih00|, (15)
1 2 1 3
ρB(t) = (α +α )|11ih11|+(α +α )|00ih00|. (16)
1 3 1 2
The results of the case of the dephasing model are given in Figs.2-??. In Fig.2, GMQD,
QD, QE and CC are plotted as a functions of the time λt for different values of system-
reservoir coupling parameterγ (namely γ/λ = 0.0,1,2) with θ = π and p = 0.5. From
60
Quantum discord and its geometric measure with death entanglement ... 9
Fig.2a, one can easily find the common features of the dynamics of GMQD, QD, QE and
CC. The previous measures of quantum and classical correlations present instantaneous
oscillations and reach their extreme values at the same time points. Because the
g
entanglement undergoes sudden death for γ 6= 0, while D , D(ρ) and Q(ρ) are long
A
g
lived. N(ρ) is not greater than D , D(ρ) and Q(ρ) for small intervals and QE attains
A
constant values during an intervals while the quantum correlations (GMQD and QD)
vary in these intervals.
In Figs.2b,3a,b, one examines the effect of system-reservoir coupling γ on the
dynamics of the previous measures of quantum and classical correlations with θ = π .
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g
It is clear that D , D(ρ) and N(ρ) decrease with increasing of γ, and they have zero
A
values. Precisely, the dephasing parameter γ leads to exponentially decay for maximum
g
values of the D , D(ρ) and N(ρ) to zero value, while Q(ρ) exponentially evolves to
A
its asymptotic value. It is interesting to note that the larger the value of γ is, the
g
more rapidly D , D(ρ) and N(ρ) reach its asymptotic values of zero. This means that,
A
quantum correlations, including entanglement and discord with its geometric measure
die asymptotically with large values of γ. On the contrary, the classical correlation
increases with increasing γ, and it have nonzero values for γ > 0. The figures show that
the classical correlation approaches an almost steady state for large values of γ faster
than that for small values of γ. Finally, after a very long time, the classical correlation
loses it oscillations and asymptotically reaches its steady state, i.e., the final state of
the qubits reaches a classical state. One can say that, the quantum states with large γ
are rapidly transformed into classical states, i.e., the processes of quantum correlation
loss and classical correlation gain are instantaneously happen.
To investigate the influence of θ = π (mixed state) with a large value of γ = 2λ on
3
the correlations see Fig.3,b. From this figure, one notes that both QD and CC reach
theirasymptoticvaluesandapproximatelyhavethesamebehaviorwhileGMQDandQE
have the same behavior. This shows that the mixedness of the initial states affects on all
the previous measures in a similar way and it inhibits them from going into zero. In
Figs.??,??, one examines the effect of the purity of the initial states on the dynamics
of the previous measures with θ = π and γ = 0.8λ. It is clear that they decrease
3
with decreasing of the purity parameter p. When the purity p is zero, all the measures
vanish, i.e, the mixedness of the initial states have the same effect on all measures. One
g
sees that the influence of purity leads to: the amplitudes of the local maxima of D ,
A
D(ρ), N(ρ) and Q(ρ) have exponential decay with decreasing the parameter p. When
quantum correlations measures quite vanishes, the states ρAB finally go into a classical
state and its quantum correlation is lost completely. This means that, after a particular
time, the purity destroys the quantum correlations of the qubits which is resulted by the
unitary interaction. Therefore, in the presence of purity one can determines a particular
region, in which, there is no state have quantum correlations.
Quantum discord and its geometric measure with death entanglement ... 10
4. Conclusions
The dynamics of quantum correlations, including entanglement and discord with its
geometric measure, and classical correlation in two-qubit models are introduced for a
open or closed quantum system. It is found that the dynamics of GMQD, QD, QE
and CC differ. Where, quantum discord and its geometric measure are exist in the
region where the entanglement is zero, which is a strong signature for the presence of
non classical correlations. System-reservoir coupling leads to: GMQD, QD and QE
die asymptotically with larger system-reservoir coupling parameter. Also, processes
of quantum correlation loss and classical correlation gain are instantaneously happen.
It is found that the purity of the initial states destroys the quantum correlations by
exponential decay. Therefore, in presence of the purity, one can determines a particular
region in which there is no state have quantum correlations.
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